1. ## Under text

How would I something to the effect of

$f(x)=x+x+x+x+...+x$
and put an under brace under the x+x+x+x+x+...+x part that says "an even number of terms}?

2. $f(x)= \underbrace{x+x+x+x+...+x}_{\text{An even number of terms}}$

Produced by:
$$f(x)= \underbrace{x+x+x+x+...+x}_{\text{An even number of terms}}$$

3. Originally Posted by o_O
$f(x)= \underbrace{x+x+x+x+...+x}_{\text{An even number of terms}}$

Produced by:
$$f(x)= \underbrace{x+x+x+x+...+x}_{\text{An even number of terms}}$$
Another tip: for best results, the ellipsis should be written using \dots = $\dots$ rather than ... = $...$. And when the ellipsis is between two operators, use \cdots to get a proper center-alignment: e.g. $1 + \frac12 + \frac13 + \cdots + \frac1n$ or $f(x) = \underbrace{x + x + x + x + \cdots + x}_{\text{An even number of terms}}$ (doesn't that look nicer?).

And thank you o_O for making me realize that there is a [noparse] tag. I used to resort to such trickery as writing [[i][/i]math]...[/tex]. This makes it much easier!

4. You can note that for a brace over the whole, it's simply \overbrace :

$f(x)=\overbrace{x+x+x+\dots+x+x}^{\text{An even number of terms}}$

$$f(x)=\overbrace{x+x+x+\dots+x+x}^{\text{An even number of terms}}$$